Shutdown of circulation pattern
could be disastrous, researchers say

Michael
Schlesinger, professor of atmospheric sciences, and
research specialist Natasha Andronova and colleagues
have co-written a report presented at the American
Geophysical Union meeting that says if global warming
shuts down the thermohaline circulation in the North
Atlantic Ocean, the result could be catastrophic climate
change.

CHAMPAIGN,
Ill. — If global warming shuts down the thermohaline circulation
in the North Atlantic Ocean, the result could be catastrophic climate
change. The environmental effects, models indicate, depend upon whether
the shutdown is reversible or irreversible.

“If the thermohaline shutdown is irreversible, we would have to
work much harder to get it to restart,” said Michael Schlesinger,
a professor of atmospheric sciences at the University of Illinois at Urbana-Champaign and a co-author of
a report to be presented at the American Geophysical Union meeting in
San Francisco, Dec. 13-17. “Not only would we have the very difficult
task of removing carbon dioxide from the atmosphere, we also would have
the virtually impossible task of removing fresh water from the North
Atlantic Ocean.”

The thermohaline circulation is driven by differences in seawater density,
caused by temperature and salinity. Like a great conveyor belt, the
circulation pattern moves warm surface water from the southern hemisphere
toward the North Pole. Between Greenland and Norway, the water cools,
sinks into the deep ocean, and begins flowing back to the south.

“This movement carries a tremendous amount of heat northward,
and plays a vital role in maintaining the current climate,” Schlesinger
said. “While shutting it down due to global warming would not
cause an ice age, as was depicted in a recent blockbuster movie, ‘The
Day After Tomorrow,’ eastern North America and western Europe
would nevertheless experience a shift in climate.”

Paleoclimate records constructed from Greenland ice cores have revealed
that the thermohaline circulation has, indeed, shut down in the past
and caused regional climate change. As the vast ice sheet that covered
much of North America during the last ice age finally receded, the meltwater
flowed out the St. Lawrence and into the North Atlantic.

“The additional fresh water made the ocean surface less dense
and it stopped sinking, effectively shutting down the thermohaline circulation,”
Schlesinger said. “As a result, Greenland cooled by about 7 degrees
Celsius within several decades. When the meltwater stopped, the circulation
pattern restarted, and Greenland warmed.”

Since the system has previously shut down by itself, “it is not
unlikely that it will do so again, especially with our help in pouring
greenhouse gases into the atmosphere,” Schlesinger said. “Higher
temperatures due to global warming could add fresh water to the northern
North Atlantic by increasing the precipitation and by melting nearby
sea ice, mountain glaciers and the Greenland ice sheet. This influx
of fresh water could reduce the surface salinity and density, leading
to a shutdown of the thermohaline circulation.

Schlesinger and his team simulated the potential effects with an uncoupled
ocean general circulation model and with it coupled to an atmosphere
general circulation model. They found that the thermohaline circulation
shut down irreversibly in the uncoupled model simulation, but reversibly
in the coupled model simulation.

“The different results occurred because of a crucial feedback
mechanism that appeared only in the coupled model simulation,”
Schlesinger said. “Enhanced evaporation increased the salinity
and density of the ocean surface, offsetting the effects of additional
fresh water.”

“The irreversible shutdown of the thermohaline circulation thus
appears to be an artifact of the model, rather than a likely outcome
of global warming,” Schlesinger said. “But, because the
possibility of an irreversible shutdown cannot be excluded, suitable
policy options should continue to be explored. Doing nothing to abate
global warming would be foolhardy if the thermohaline circulation shutdown
is irreversible.”

Coauthors are U. of I. graduate student Jianjun Yin, research specialist
Natasha Andronova, research programmer Bin Li, and Princeton University
researcher Sergey Malyshev. The National Science Foundation funded the
work.